Method b requires the volta address to have authorized this call.
When Volta's multisig invokes Contract A, and A calls B.b,
the Volta contract must provide auth_entries so that
volta.require_auth() succeeds.
fn b(env: soroban_sdk::Env, volta: soroban_sdk::Address, value: u64) -> u64
fn get_volta_value(env: soroban_sdk::Env, volta: soroban_sdk::Address) -> Option